1. Field of the Invention
This invention relates to methods for measuring the temperature of a semiconductor wafer.
2. Description of the Related Art
In semiconductor device fabrication, the temperature of the wafer and temperature uniformity across the wafer during processing are important parameters. Semiconductor material properties such as stress, dielectric constant, density, and resistivity depend on the wafer processing temperature. Thus, it is critical for the semiconductor manufacturing equipment to maintain the wafer's temperature during processing at the level specified in the process recipe. Uncontrolled wafer temperature will result in material properties shift, thereby rendering the device defective.
In typical semiconductor manufacturing equipment, such as chemical vapor deposition (CVD) reactors, the wafer's temperature during processing can be determined indirectly by measuring the chuck or pedestal temperature. As shown in FIG. 1, wafer 1 is supported by pedestal 2 during processing in a reactor. To improve heat transfer, helium or argon gas is flowed in-between wafer 1 and pedestal 2. Temperature sensor 3, which could be a thermocouple, measures the temperature of pedestal 2. The output of temperature sensor 3 is used by temperature controller 4 to determine if adjustments are required to maintain the temperature of wafer 1 within the range specified in the process recipe. Because the temperature of wafer 1 is not being measured directly, the temperature of pedestal 2 is assumed to be the same as that of wafer 1. In reality, however, the temperature difference between wafer 1 and pedestal 2 could range from 10.degree. to 50.degree. C. because of inefficient heat transfer.
In high density plasma chemical vapor deposition (HDP CVD) reactors, such as those manufactured by Applied Materials Inc. of Santa Clara, Calif. and Novellus Systems of San Jose, Calif., the problems associated with measuring pedestal temperature to determine wafer temperature are exacerbated. In conventional CVD reactors, heat is transferred from the pedestal to the backside of the wafer. In contrast, in HDP CVD reactors, heat is transferred from the plasma, to the active side of the wafer, and then to the pedestal. Thus, in an HDP CVD reactor, the top or active side of the wafer is at a higher temperature than the pedestal and depends on various parameters such as RF power, reactant gas flows, backside gas flows, and chamber pressure, making it more difficult to determine the wafer's process temperature by simply taking measurements on the pedestal.
Measuring wafer process temperature directly is not only costly but also poses its own set of technical difficulties. One way of measuring wafer temperature directly is to use a pyrometer. To employ a pyrometer, however, requires that the backside emissivity of the wafer be accurately and precisely modeled; a task which is costly, error-prone, and time consuming. Another way of directly measuring the temperature of the wafer during processing is to attach a thermocouple on the backside of the wafer. Direct temperature measurement using a thermocouple does not give accurate readings because reliable and repeatable physical connection to the backside of the wafer is necessary. Further, for both the pyrometer and thermocouple methods, only the temperature of a single point on the wafer is measured. Measuring temperature uniformity across the wafer using a pyrometer or thermocouple is costly, difficult, and practically unfeasible. And because pyrometer and thermocouple temperature measurements are taken on the backside of the wafer while the actual devices are on the top or active side, there is an inherent inaccuracy in the measurement. Positioning sensors to allow taking temperature measurements on the active side of the wafer is often times difficult, if not impossible, because the wafer's active side is exposed to plasma and process gases.
Thus, there is a clear need for an accurate, repeatable, and practical method for determining the wafer's temperature and temperature uniformity during semiconductor processing.